1. Field of the Invention
The present invention relates to a flashing apparatus of an automatic light-adjusting type for a camera generally called an auto-stroboscope.
2. Description of the Related Art
In a general flashing apparatus of this kind, light is emitted from a light emitting tube and light reflected from a photographed object is integrated. When the integrated light has reached a predetermined value, the light emission of the light emitting tube is stopped by flowing the charge of a main capacitor through a bypass line. Such a parallel control system is described in U.S. Pat. No. 3,033,988 patented in 1957. In a so-called series control system, when the light reflected from the photographed object is integrated and has reached a predetermined value, a switching element connected in series to the light emitting tube is turned off to stop the light emission thereof. This series control system is described in Japanese Patent Publication (KOKOKU) No. 44-30905.
Further, there is a known system for controlling a diaphragmof a camera by detecting the voltage of a main capacitor. Furthermore, there is a known system for controlling a guide number by changing a light emitting time. Such control systems are not used for an auto-stroboscope.
Recently, cameras using zoom lenses have rapidly spread so that the number of stroboscopes for illumination corresponding to the zoom lenses is increased. When a stroboscope using a zoom lens is used, a light emitting capacity is changed in accordance with a position of the zoom lens. Namely, when the same energy is discharged by a discharge tube to emit light therefrom, a guide number at a telescopic time is larger than that at a wide angle time. Therefore, it is necessary to change an amount of energy supplied to the discharge tube at the same distance from the camera to the photographed object in accordance with the wide angle and telescopic cases.
In the general stroboscope of the series control system, a known stroboscopic controller using an IGBT (Insulated Gate Bipolar Transistor) is disposed to simplify the structure of the stroboscope. In particular, the stroboscopic controller has advantages that it is not necessary to dispose a circuit for applying a backward bias voltage at a stopping time of the operation of the stroboscope in comparison with series control performed by a thyristor often used in the stroboscopic controller at the present time. Further, no flashover is caused in this stroboscopic controller.
When light is emitted from the stroboscope, a peak electric current of the stroboscope often ranges from 100 to 250 A in the case of a camera. This peak electric current depends on a voltage of the main capacitor and an impedance of a xenon lamp for discharge, etc. When this peak electric current flows through the stroboscope, it is necessary to apply a limited voltage about 30 to 40 V to a gate of the insulated gate bipolar transistor. In a certain kind of the insulated gate bipolar transistor, it is necessary to apply a limited voltage of about 15 to 25 V to a gate of the insulated gate bipolar transistor.
Accordingly, it is desirable that the voltage applied to the gate of the insulated gate bipolar transistor is constant in a range in which the peak electric current can flow through the insulated gate bipolar transistor.
Japanese Patent Application Laying Open (KOKAI) Nos. 64-17033 and 1-124838 show stroboscopes using the insulated gate bipolar transistor.
However, the stroboscope having a structure for receiving light reflected from the photographed object has the following problems.
(1) The light emission of the stroboscope is stopped by receiving the light reflected from the photographed object. Therefore, it is impossible to adjust a quantity of received light when the photographed object is too far from the stroboscope or too near the stroboscope.
(2) The amount of the received light is changed in accordance with the reflectivity of the photographed object so that exposure is changed. For example, exposure is excessively increased when a background of the photographed object has a low reflectivity. In contrast to this, exposure is excessively decreased when the background of the photographed object has a high reflectivity.
(3) An error in exposure is caused in a TTL direct system in accordance with a difference in reflectivity between film faces.
(4) It is necessary that a light-receiving element has a high sensitivity and a high responsive speed. Further, it is necessary to dispose many parts requiring high accuracy in operation such as a light quantity integrating circuit, an integrating capacitor, etc., thereby increasing the cost of the stroboscope and a space therefor.
The stroboscope for performing a control operation by changing a light emitting time has the following problems.
(1) The light emitting time of the stroboscope is short so that it is difficult to control the operation of the stroboscope with high accuracy.
(2) The quantity of light is greatly changed in accordance with the voltage of a main capacitor just before light is emitted from the stroboscope.
(3) It is difficult to control a guide number when the guide number is small at a small quantity of light.
In the camera using a general zoom stroboscope, it is necessary to change the diameter of a diaphragm in accordance with zooming information. Therefore, it is necessary to associate the operation of the stroboscope with the operation of a mechanical mechanism for operating the camera. Accordingly, the accuracy in control of the quantity of light cannot be easily improved and the mechanism is very complicated.
Further, in the general stroboscope, when no voltage about 30 to 40 V, or 15 to 25 V is applied to the gate of the insulated gate bipolar transistor so as to operate the insulated gate bipolar transistor, it is impossible to provide a predetermined quantity of emitted light and the insulated gate bipolar transistor is broken in a certain case.
In Japanese Patent Application Laying Open (KOKAI) Nos. 64-17033 and 1-124838 mentioned above, there is no concrete description about a system for stopping the light emission. For example, when a known circuit for integrating light reflected from the photographed object is used in the stroboscope, it is necessary to dispose many parts requiring high speed and accuracy in operation. Accordingly, it is impossible to greatly reduce the cost of the stroboscope and make constructional circuits thereof compact even when the insulated gate bipolar transistor is used in the stroboscope.
When a method for controlling the light emitting time is used, the constructional circuits of the stroboscope are made compact. However, in this case, a small quantity of light cannot be emitted from the stroboscope with high accuracy. Further, it is impossible to preferably perform a light emitting control operation in a wide range at a certain voltage of the main capacitor at the light emitting time of the stroboscope.